Direct memory access transfer completion notification

ABSTRACT

Methods, compute nodes, and computer program products are provided for direct memory access (‘DMA’) transfer completion notification. Embodiments include determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer; and notifying a processor core on the origin DMA engine that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No. B554331 awarded by the Department of Energy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is data processing, or, more specifically, methods, compute nodes, and products for direct memory access (‘DMA’) transfer completion notification.

2. Description of Related Art

The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. Today's computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago.

Parallel computing is an area of computer technology that has experienced advances. Parallel computing is the simultaneous execution of the same task (split up and specially adapted) on multiple processors in order to obtain results faster. Parallel computing is based on the fact that the process of solving a problem usually can be divided into smaller tasks, which may be carried out simultaneously with some coordination.

Parallel computers execute parallel algorithms. A parallel algorithm can be split up to be executed a piece at a time on many different processing devices, and then put back together again at the end to get a data processing result. Some algorithms are easy to divide up into pieces. Splitting up the job of checking all of the numbers from one to a hundred thousand to see which are primes could be done, for example, by assigning a subset of the numbers to each available processor, and then putting the list of positive results back together. In this specification, the multiple processing devices that execute the individual pieces of a parallel program are referred to as ‘compute nodes.’ A parallel computer is composed of compute nodes and other processing nodes as well, including, for example, input/output (‘I/O’) nodes, and service nodes.

Parallel algorithms are valuable because it is faster to perform some kinds of large computing tasks via a parallel algorithm than it is via a serial (non-parallel) algorithm, because of the way modem processors work. It is far more difficult to construct a computer with a single fast processor than one with many slow processors with the same throughput. There are also certain theoretical limits to the potential speed of serial processors. On the other hand, every parallel algorithm has a serial part and so parallel algorithms have a saturation point. After that point adding more processors does not yield any more throughput but only increases the overhead and cost.

Parallel algorithms are designed also to optimize one more resource the data communications requirements among the nodes of a parallel computer. There are two ways parallel processors communicate, shared memory or message passing. Shared memory processing needs additional locking for the data and imposes the overhead of additional processor and bus cycles and also serializes some portion of the algorithm.

Message passing processing uses high-speed data communications networks and message buffers, but this communication adds transfer overhead on the data communications networks as well as additional memory need for message buffers and latency in the data communications among nodes. Designs of parallel computers use specially designed data communications links so that the communication overhead will be small but it is the parallel algorithm that decides the volume of the traffic.

Many data communications network architectures are used for message passing among nodes in parallel computers. Compute nodes may be organized in a network as a ‘torus’ or ‘mesh,’ for example. Also, compute nodes may be organized in a network as a tree. A torus network connects the nodes in a three-dimensional mesh with wrap around links. Every node is connected to its six neighbors through this torus network, and each node is addressed by its x,y,z coordinate in the mesh. In a tree network, the nodes typically are connected into a binary tree: each node has a parent, and two children (although some nodes may only have zero children or one child, depending on the hardware configuration). In computers that use a torus and a tree network, the two networks typically are implemented independently of one another, with separate routing circuits, separate physical links, and separate message buffers.

A torus network lends itself to point to point operations, but a tree network typically is inefficient in point to point communication. A tree network, however, does provide high bandwidth and low latency for certain collective operations, message passing operations where all compute nodes participate simultaneously, such as, for example, an allgather.

When a parallel application running on a compute node transmits a message to another compute node connected to the data communications network, the application typically provides messaging software with a pointer to an application buffer storing the message. The messaging software describes the message in the application buffer to a direct memory access (‘DMA’) engine, which in turn, sends the message through the network using a DMA transfer. The drawback to this current method of transmitting messages, however, is that the application must typically monitor for the completion of the transfer to free up the application buffer for use with another operation after the transfer is complete regardless of whether the application needs to know that the transfer is complete. Having the application needlessly monitor for completion of the transfer increases the processing overhead and resources required to effect message transmission.

SUMMARY OF THE INVENTION

Methods, compute nodes, and computer program products are provided for direct memory access (‘DMA’) transfer completion notification. Embodiments include determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer; and notifying a processor core on the origin DMA engine that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system for direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention.

FIG. 2 sets forth a block diagram of an exemplary compute node useful in a parallel computer capable of direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention.

FIG. 3A illustrates an exemplary Point To Point Adapter useful in systems capable of direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention.

FIG. 3B illustrates an exemplary Global Combining Network Adapter useful in systems capable of direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention.

FIG. 4 sets forth a line drawing illustrating an exemplary data communications network optimized for point to point operations useful in systems capable of direct memory access (‘DMA’) transfer completion notification in accordance with embodiments of the present invention.

FIG. 5 sets forth a line drawing illustrating an exemplary data communications network optimized for collective operations useful in systems capable of direct memory access (‘DMA’) transfer completion notification in accordance with embodiments of the present invention.

FIG. 6 sets forth a block diagram illustrating an exemplary communications architecture illustrated as a protocol stack useful in direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention.

FIG. 7 sets forth a flow chart illustrating an exemplary method for direct memory access (‘DMA’) transfer completion notification according to the present invention.

FIG. 8 sets forth a flow chart illustrating an exemplary method for determining, by the origin DMA engine, whether a data descriptor for a message is currently in the injection FIFO buffer.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary methods, apparatus, and computer program products for direct memory access (‘DMA’) transfer completion notification according to embodiments of the present invention are described with reference to the accompanying drawings, beginning with FIG. 1. FIG. 1 illustrates an exemplary system for DMA transfer completion notification according to embodiments of the present invention. The system of FIG. 1 includes a parallel computer (100), non-volatile memory for the computer in the form of data storage device (118), an output device for the computer in the form of printer (120), and an input/output device for the computer in the form of computer terminal (122). Parallel computer (100) in the example of FIG. 1 includes a plurality of compute nodes (102).

The compute nodes (102) are coupled for data communications by several independent data communications networks including a high speed Ethernet network (174), a Joint Test Action Group (‘JTAG’) network (104), a global combining network (106) which is optimized for collective operations, and a torus network (108) which is optimized point to point operations. The global combining network (106) is a data communications network that includes data communications links connected to the compute nodes so as to organize the compute nodes as a tree. Each data communications network is implemented with data communications links among the compute nodes (102). The data communications links provide data communications for parallel operations among the compute nodes of the parallel computer.

In addition, the compute nodes (102) of parallel computer are organized into at least one operational group (132) of compute nodes for collective parallel operations on parallel computer (100). An operational group of compute nodes is the set of compute nodes upon which a collective parallel operation executes. Collective operations are implemented with data communications among the compute nodes of an operational group. Collective operations are those functions that involve all the compute nodes of an operational group. A collective operation is an operation, a message-passing computer program instruction that is executed simultaneously, that is, at approximately the same time, by all the compute nodes in an operational group of compute nodes. Such an operational group may include all the compute nodes in a parallel computer (100) or a subset all the compute nodes. Collective operations are often built around point to point operations. A collective operation requires that all processes on all compute nodes within an operational group call the same collective operation with matching arguments. A ‘broadcast’ is an example of a collective operation for moving data among compute nodes of an operational group. A ‘reduce’ operation is an example of a collective operation that executes arithmetic or logical functions on data distributed among the compute nodes of an operational group. An operational group may be implemented as, for example, an MPI ‘communicator.’

‘MPI’ refers to ‘Message Passing Interface,’ a prior art parallel communications library, a module of computer program instructions for data communications on parallel computers. Examples of prior-art parallel communications libraries that may be improved for use with systems according to embodiments of the present invention include MPI and the ‘Parallel Virtual Machine’ (‘PVM’) library. PVM was developed by the University of Tennessee, The Oak Ridge National Laboratory, and Emory University. MPI is promulgated by the MPI Forum, an open group with representatives from many organizations that define and maintain the MPI standard. MPI at the time of this writing is a de facto standard for communication among compute nodes running a parallel program on a distributed memory parallel computer. This specification sometimes uses MPI terminology for ease of explanation, although the use of MPI as such is not a requirement or limitation of the present invention.

Some collective operations have a single originating or receiving process running on a particular compute node in an operational group. For example, in a ‘broadcast’ collective operation, the process on the compute node that distributes the data to all the other compute nodes is an originating process. In a ‘gather’ operation, for example, the process on the compute node that received all the data from the other compute nodes is a receiving process. The compute node on which such an originating or receiving process runs is referred to as a logical root.

Most collective operations are variations or combinations of four basic operations: broadcast, gather, scatter, and reduce. The interfaces for these collective operations are defined in the MPI standards promulgated by the MPI Forum. Algorithms for executing collective operations, however, are not defined in the MPI standards. In a broadcast operation, all processes specify the same root process, whose buffer contents will be sent. Processes other than the root specify receive buffers. After the operation, all buffers contain the message from the root process.

In a scatter operation, the logical root divides data on the root into segments and distributes a different segment to each compute node in the operational group. In scatter operation, all processes typically specify the same receive count. The send arguments are only significant to the root process, whose buffer actually contains sendcount * N elements of a given data type, where N is the number of processes in the given group of compute nodes. The send buffer is divided and dispersed to all processes (including the process on the logical root). Each compute node is assigned a sequential identifier termed a ‘rank.’ After the operation, the root has sent sendcount data elements to each process in increasing rank order. Rank 0 receives the first sendcount data elements from the send buffer. Rank 1 receives the second sendcount data elements from the send buffer, and so on.

A gather operation is a many-to-one collective operation that is a complete reverse of the description of the scatter operation. That is, a gather is a many-to-one collective operation in which elements of a datatype are gathered from the ranked compute nodes into a receive buffer in a root node.

A reduce operation is also a many-to-one collective operation that includes an arithmetic or logical function performed on two data elements. All processes specify the same ‘count’ and the same arithmetic or logical function. After the reduction, all processes have sent count data elements from computer node send buffers to the root process. In a reduction operation, data elements from corresponding send buffer locations are combined pair-wise by arithmetic or logical operations to yield a single corresponding element in the root process's receive buffer. Application specific reduction operations can be defined at runtime. Parallel communications libraries may support predefined operations. MPI, for example, provides the following predefined reduction operations:

MPI_MAX maximum MPI_MIN minimum MPI_SUM sum MPI_PROD product MPI_LAND logical and MPI_BAND bitwise and MPI_LOR logical or MPI_BOR bitwise or MPI_LXOR logical exclusive or MPI_BXOR bitwise exclusive or

In addition to compute nodes, the parallel computer (100) includes input/output (‘I/O’) nodes (110, 114) coupled to compute nodes (102) through one of the data communications networks (174). The I/O nodes (110, 114) provide I/O services between compute nodes (102) and I/O devices (118, 120, 122). I/O nodes (110, 114) are connected for data communications I/O devices (118, 120, 122) through local area network (‘LAN’) (130). The parallel computer (100) also includes a service node (116) coupled to the compute nodes through one of the networks (104). Service node (116) provides service common to pluralities of compute nodes, loading programs into the compute nodes, starting program execution on the compute nodes, retrieving results of program operations on the computer nodes, and so on. Service node (116) runs a service application (124) and communicates with users (128) through a service application interface (126) that runs on computer terminal (122).

As described in more detail below in this specification, the system of FIG. 1 operates generally for DMA transfer completion notification according to embodiments of the present invention according to embodiments of the present invention. The system of FIG. 1 operates generally for DMA transfer completion notification according to embodiments of the present invention by determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer; and notifying a processor core on the origin DMA engine that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer. Readers will note that the origin compute node is a compute node attempting to transmit a message, while the target compute node is a compute node intended as the recipient of the message.

The arrangement of nodes, networks, and I/O devices making up the exemplary system illustrated in FIG. 1 are for explanation only, not for limitation of the present invention. Data processing systems capable of DMA transfer completion notification according to embodiments of the present invention may include additional nodes, networks, devices, and architectures, not shown in FIG. 1, as will occur to those of skill in the art. Although the parallel computer (100) in the example of FIG. 1 includes sixteen compute nodes (102), readers will note that parallel computers capable of DMA transfer completion notification according to embodiments of the present invention may include any number of compute nodes. In addition to Ethernet and JTAG, networks in such data processing systems may support many data communications protocols including for example TCP (Transmission Control Protocol), IP (Internet Protocol), and others as will occur to those of skill in the art. Various embodiments of the present invention may be implemented on a variety of hardware platforms in addition to those illustrated in FIG. 1.

DMA transfer completion notification according to embodiments of the present invention may be generally implemented on a parallel computer that includes a plurality of compute nodes. In fact, such computers may include thousands of such compute nodes. Each compute node is in turn itself a kind of computer composed of one or more computer processors, its own computer memory, and its own input/output adapters. For further explanation, therefore, FIG. 2 sets forth a block diagram of an exemplary compute node useful in a parallel computer capable of DMA transfer completion notification according to embodiments of the present invention. The compute node (152) of FIG. 2 includes one or more computer processors (164) as well as random access memory (‘RAM’) (156). The processors (164) are connected to RAM (156) through a high-speed memory bus (154) and through a bus adapter (194) and an extension bus (168) to other components of the compute node (152). Stored in RAM (156) is an application program (158), a module of computer program instructions that carries out parallel, user-level data processing using parallel algorithms. The application (158) of FIG. 2 allocates an application buffer for storing a message for transmission to another compute node.

Also stored in RAM (156) is a messaging module (160), a library of computer program instructions that carry out parallel communications among compute nodes, including point to point operations as well as collective operations. Application program (158) executes collective operations by calling software routines in the messaging module (160). A library of parallel communications routines may be developed from scratch for use in systems according to embodiments of the present invention, using a traditional programming language such as the C programming language, and using traditional programming methods to write parallel communications routines that send and receive data among nodes on two independent data communications networks. Alternatively, existing prior art libraries may be improved to operate according to embodiments of the present invention. Examples of prior-art parallel communications libraries include the ‘Message Passing Interface’ (‘MPI’) library and the ‘Parallel Virtual Machine’ (‘PVM’) library.

Also stored in RAM (156) is an operating system (162), a module of computer program instructions and routines for an application program's access to other resources of the compute node. It is typical for an application program and parallel communications library in a compute node of a parallel computer to run a single thread of execution with no user login and no security issues because the thread is entitled to complete access to all resources of the node. The quantity and complexity of tasks to be performed by an operating system on a compute node in a parallel computer therefore are smaller and less complex than those of an operating system on a serial computer with many threads running simultaneously. In addition, there is no video I/O on the compute node (152) of FIG. 2, another factor that decreases the demands on the operating system. The operating system may therefore be quite lightweight by comparison with operating systems of general purpose computers, a pared down version as it were, or an operating system developed specifically for operations on a particular parallel computer. Operating systems that may usefully be improved, simplified, for use in a compute node include UNIX™, Linux™, Microsoft XP™, AIX™, IBM's i5/OS™, and others as will occur to those of skill in the art.

The exemplary compute node (152) of FIG. 2 includes several communications adapters (172, 176, 180, 188) for implementing data communications with other nodes of a parallel computer. Such data communications may be carried out serially through RS-232 connections, through external buses such as USB, through data communications networks such as IP networks, and in other ways as will occur to those of skill in the art. Communications adapters implement the hardware level of data communications through which one computer sends data communications to another computer, directly or through a network. Examples of communications adapters useful in systems for DMA transfer completion notification according to embodiments of the present invention include modems for wired communications, Ethernet (IEEE 802.3) adapters for wired network communications, and 802.11b adapters for wireless network communications.

The data communications adapters in the example of FIG. 2 include a Gigabit Ethernet adapter (172) that couples example compute node (152) for data communications to a Gigabit Ethernet (174). Gigabit Ethernet is a network transmission standard, defined in the IEEE 802.3 standard, that provides a data rate of 1 billion bits per second (one gigabit). Gigabit Ethernet is a variant of Ethernet that operates over multimode fiber optic cable, single mode fiber optic cable, or unshielded twisted pair.

The data communications adapters in the example of FIG. 2 includes a JTAG Slave circuit (176) that couples example compute node (152) for data communications to a JTAG Master circuit (178). JTAG is the usual name used for the IEEE 1149.1 standard entitled Standard Test Access Port and Boundary-Scan Architecture for test access ports used for testing printed circuit boards using boundary scan. JTAG is so widely adapted that, at this time, boundary scan is more or less synonymous with JTAG. JTAG is used not only for printed circuit boards, but also for conducting boundary scans of integrated circuits, and is also useful as a mechanism for debugging embedded systems, providing a convenient “back door” into the system. The example compute node of FIG. 2 may be all three of these: It typically includes one or more integrated circuits installed on a printed circuit board and may be implemented as an embedded system having its own processor, its own memory, and its own I/O capability. JTAG boundary scans through JTAG Slave (176) may efficiently configure processor registers and memory in compute node (152) for use with DMA transfer completion notification according to embodiments of the present invention.

The data communications adapters in the example of FIG. 2 includes a Point To Point Adapter (180) that couples example compute node (152) for data communications to a network (108) that is optimal for point to point message passing operations such as, for example, a network configured as a three-dimensional torus or mesh. Point To Point Adapter (180) provides data communications in six directions on three communications axes, x, y, and z, through six bidirectional links: +x (181), −x (182), +y (183), −y (184), +z (185), and −z (186).

The data communications adapters in the example of FIG. 2 includes a Global Combining Network Adapter (188) that couples example compute node (152) for data communications to a network (106) that is optimal for collective message passing operations on a global combining network configured, for example, as a binary tree. The Global Combining Network Adapter (188) provides data communications through three bidirectional links: two to children nodes (190) and one to a parent node (192).

Example compute node (152) includes two arithmetic logic units (‘ALUs’). ALU (166) is a component of processor (164), and a separate ALU (170) is dedicated to the exclusive use of Global Combining Network Adapter (188) for use in performing the arithmetic and logical functions of reduction operations. Computer program instructions of a reduction routine in parallel communications library (160) may latch an instruction for an arithmetic or logical function into instruction register (169). When the arithmetic or logical function of a reduction operation is a ‘sum’ or a ‘logical or,’ for example, Global Combining Network Adapter (188) may execute the arithmetic or logical operation by use of ALU (166) in processor (164) or, typically much faster, by use dedicated ALU (170).

The example compute node (152) of FIG. 2 includes a direct memory access (‘DMA’) controller (195), which is computer hardware for direct memory access and a DMA engine (197), which is computer software for direct memory access. Direct memory access includes reading and writing to memory of compute nodes with reduced operational burden on the central processing units (164). A DMA transfer essentially copies a block of memory from one compute node to another. While the CPU may initiate the DMA transfer, the CPU does not execute it. The DMA engine (195) of FIG. 2 is improved for DMA transfer completion notification according to embodiments of the present invention. The DMA engine (197) of FIG. 2 operates generally for DMA transfer completion notification by determining whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer; and notifying a processor core on the origin DMA engine that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.

For further explanation, FIG. 3A illustrates an exemplary Point To Point Adapter (180) useful in systems capable of DMA transfer completion notification according to embodiments of the present invention. Point To Point Adapter (180) is designed for use in a data communications network optimized for point to point operations, a network that organizes compute nodes in a three-dimensional torus or mesh. Point To Point Adapter (180) in the example of FIG. 3A provides data communication along an x-axis through four unidirectional data communications links, to and from the next node in the −x direction (182) and to and from the next node in the +x direction (181). Point To Point Adapter (180) also provides data communication along a y-axis through four unidirectional data communications links, to and from the next node in the −y direction (184) and to and from the next node in the +y direction (183). Point To Point Adapter (180) in FIG. 3A also provides data communication along a z-axis through four unidirectional data communications links, to and from the next node in the −z direction (186) and to and from the next node in the +z direction (185).

For further explanation, FIG. 3B illustrates an exemplary Global Combining Network Adapter (188) useful in systems capable of DMA transfer completion notification according to embodiments of the present invention. Global Combining Network Adapter (188) is designed for use in a network optimized for collective operations, a network that organizes compute nodes of a parallel computer in a binary tree. Global Combining Network Adapter (188) in the example of FIG. 3B provides data communication to and from two children nodes through four unidirectional data communications links (190). Global Combining Network Adapter (188) also provides data communication to and from a parent node through two unidirectional data communications links (192).

For further explanation, FIG. 4 sets forth a line drawing illustrating an exemplary data communications network (108) optimized for point to point operations useful in systems capable of that DMA transfer completion notification in accordance with embodiments of the present invention. In the example of FIG. 4, dots represent compute nodes (102) of a parallel computer, and the dotted lines between the dots represent data communications links (103) between compute nodes. The data communications links are implemented with point to point data communications adapters similar to the one illustrated for example in FIG. 3A, with data communications links on three axes, x, y, and z, and to and fro in six directions +x (181), −x (182), +y (183), −y (184), +z (185), and −z (186). The links and compute nodes are organized by this data communications network optimized for point to point operations into a three dimensional mesh (105). The mesh (105) has wrap-around links on each axis that connect the outermost compute nodes in the mesh (105) on opposite sides of the mesh (105). These wrap-around links form part of a torus (107). Each compute node in the torus has a location in the torus that is uniquely specified by a set of x, y, z coordinates. Readers will note that the wrap-around links in the y and z directions have been omitted for clarity, but are configured in a similar manner to the wrap-around link illustrated in the x direction. For clarity of explanation, the data communications network of FIG. 4 is illustrated with only 27 compute nodes, but readers will recognize that a data communications network optimized for point to point operations for use in DMA transfer completion notification in accordance with embodiments of the present invention may contain only a few compute nodes or may contain thousands of compute nodes.

For further explanation, FIG. 5 sets forth a line drawing illustrating an exemplary data communications network (106) optimized for collective operations useful in systems capable of DMA transfer completion notification in accordance with embodiments of the present invention. The example data communications network of FIG. 5 includes data communications links connected to the compute nodes so as to organize the compute nodes as a tree. In the example of FIG. 5, dots represent compute nodes (102) of a parallel computer, and the dotted lines (103) between the dots represent data communications links between compute nodes. The data communications links are implemented with global combining network adapters similar to the one illustrated for example in FIG. 3B, with each node typically providing data communications to and from two children nodes and data communications to and from a parent node, with some exceptions. Nodes in a binary tree (106) may be characterized as a physical root node (202), branch nodes (204), and leaf nodes (206). The root node (202) has two children but no parent. The leaf nodes (206) each has a parent, but leaf nodes have no children. The branch nodes (204) each has both a parent and two children. The links and compute nodes are thereby organized by this data communications network optimized for collective operations into a binary tree (106). For clarity of explanation, the data communications network of FIG. 5 is illustrated with only 31 compute nodes, but readers will recognize that a data communications network optimized for collective operations for use in systems capable of DMA transfer completion notification accordance with embodiments of the present invention may contain only a few compute nodes or may contain thousands of compute nodes.

In the example of FIG. 5, each node in the tree is assigned a unit identifier referred to as a ‘rank’ (250). A node's rank uniquely identifies the node's location in the tree network for use in both point to point and collective operations in the tree network. The ranks in this example are assigned as integers beginning with 0 assigned to the root node (202), 1 assigned to the first node in the second layer of the tree, 2 assigned to the second node in the second layer of the tree, 3 assigned to the first node in the third layer of the tree, 4 assigned to the second node in the third layer of the tree, and so on. For ease of illustration, only the ranks of the first three layers of the tree are shown here, but all compute nodes in the tree network are assigned a unique rank.

For further explanation, FIG. 6 sets forth a block diagram illustrating an exemplary communications architecture illustrated as a protocol stack useful in DMA transfer completion notification according to embodiments of the present invention. The exemplary communications architecture of FIG. 6 sets forth two compute nodes, origin compute node (600) and target compute node (604). Only two compute nodes are illustrated in the example of FIG. 6 for ease of explanation and not for limitation. In fact, DMA transfer completion notification according to embodiments of the present invention may be implemented using many compute nodes in very large scale computer systems such as parallel computers with thousands of nodes.

The exemplary communications architecture of FIG. 6 includes an application layer (602) composed of application (158) installed on the origin compute node (600) and application (606) installed on the target compute node (604). In the example of FIG. 6, the application (158) includes an application buffer (608) for storing a message (614) for transmission to application (606) installed on the target compute node (604). Data communications between applications (158, 606) are effected using messaging modules (160, 612) installed on each of the compute nodes (600, 604). Applications (158) may communicate messages by invoking function of an application programming interfaces (‘API’) exposed by the application messaging modules (606 and 612). To transmit message (614) to the application (606), the application (158) of FIG. 6 may invoke a function of an API for messaging module (160) that passes a buffer identifier specifying the application buffer (750) to the messaging module (160).

The exemplary communications architecture of FIG. 6 includes a messaging layer (610) that implements data communications protocols for data communications that support messaging in the application layer (602). In the example of FIG. 6, the messaging layer (610) is composed of messaging module (160) installed on the origin compute node (600) and messaging module (612) installed on the target compute node (604) and the messaging modules care capable of operating with DMA transfer completion notification according to embodiments of the present invention.

Such data communications protocols of the messaging layer are typically invoked through a set of APIs that are exposed to the applications (158 and 606) in the application layer (602). When an application invokes an API for the messaging module, the messaging module (160) of FIG. 6 receives a buffer identifier specifying the application buffer (750) having a message (614) for transmission to a target compute node (604) through a data communications network (108). The messaging module (160) then creates a data descriptor (706) that specifies the message of the DMA transfer operation for transferring the application message (601) from the origin compute node (600) to the target compute node (604). A data descriptor (706) for a message (601) is a data structure that describes the message for transmission and specifies transmission data related to that message. For example, a data descriptor may describe the address of the buffer storing the message, the size of the message, the type of the message, packet headers to be used in transmitting the message, and other information as will occur to those of skill in the art.

The exemplary communications architecture of FIG. 6 includes a hardware layer (634) that defines the physical implementation and the electrical implementation of aspects of the hardware on the compute nodes such as the bus, network cabling, connector types, physical data rates, data transmission encoding and may other factors for communications between the compute nodes (600 and 604) on the physical network medium. The hardware layer (634) of FIG. 6 is composed of communications hardware (636) of the origin compute node (600), communications hardware (638) of the target compute node (636), and the data communications network (108) connecting the origin compute node (600) to the target compute node (604). Such communications hardware may include, for example, point-to-point adapters as described above with reference to FIGS. 2 and 3A.

The exemplary communications architecture of FIG. 6 illustrates a DMA engine (197) for the origin compute node (600). The DMA engine (197) in the example of FIG. 6 is illustrated in both the messaging module layer (610) and the hardware layer (634). The DMA engine (197) is shown in both the messaging layer (610) and the hardware layer (634) because a DMA engine useful in DMA transfer completion notification according to embodiments of the present invention may often provide messaging layer interfaces and also implement communications according to some aspects of the communication hardware layer (634). The exemplary DMA engine (197) of FIG. 6 includes an injection FIFO buffer (628) for storing data descriptors for messages to be sent to other DMA engines on other compute nodes using a memory FIFO data transfer operation or direct put data transfer operation. Although not shown, the exemplary DMA engine (197) of FIG. 6 also includes a number of reception FIFO buffers for storing data descriptors for messages received from other DMA engines on other compute nodes. Although FIG. 6 only illustrates a single injection FIFO buffer (628), readers will note that a DMA engine may have access to any number of injection FIFO buffers and reception FIFO buffers.

A memory FIFO data transfer operation is a mode of transferring data using a DMA engine on an origin node and a DMA engine on a target node. In a memory FIFO data transfer operation, data is transferred along with a data descriptor describing the data from an injection FIFO for the origin DMA engine to a target DMA engine. The target DMA engine in turns places the descriptor in the reception FIFO and caches the data. A core processor then retrieves the data descriptor from the reception FIFO and processes the data in cache either by instructing the DMA to store the data directly or carrying out some processing on the data, such as even storing the data by the core processor.

A direct put operation is a mode of transferring data using a DMA engine on an origin node and a DMA engine on a target node. A direct put operation allows data to be transferred and stored on the target compute node with little or no involvement from the target node's processor. To effect minimal involvement from the target node's processor in the direct put operation, the origin DMA transfers the data to be stored on the target compute node along with a specific identification of a storage location on the target compute node. The origin DMA knows the specific storage location on the target compute node because the specific storage location for storing the data on the target compute node has been previously provided by the target DMA to the origin DMA.

In addition to specifying DMA data transfer operations, each data descriptor in the injection FIFO buffer (628) of FIG. 6 also specifies one of the shared DMA counters (616) for use in the DMA transfer operation for transferring the message. A shared DMA counter is a DMA counter that is utilized concurrently for multiple data transfer operations. That is, more than one descriptor in the injection FIFO buffer (628) may specify the same counter (616). The use of shared DMA counters helps to avoid the problem of having to suspend data transfers until a dedicated DMA counter is available to be assigned to a data descriptor (618) for use in a DMA transfer operation. Dedicated DMA counters for each data transfer operation may not always be available because of the limited number of DMA counters typically allocated to the DMA engine. In fact, in many system configurations, the DMA engine may only have access to sixty-four DMA counters.

The shared DMA counters (616) of FIG. 6 differ from traditional dedicated DMA counters which are not shared concurrently among multiple data descriptors. Such traditional dedicated DMA counters are used by the origin compute node to identify when a data transfer is complete. In such traditional systems, the DMA engine uses a dedicated DMA counter associated with a particular data descriptor to track the amount of message data remaining to be sent for that descriptor by setting the value of the counter to the size of the data to be transferred and decrementing the value of the counter as the data is transferred to the target compute node.

As mentioned above, the DMA counters (616) used by the DMA engine (197) in the example of FIG. 6 are shared DMA counters and do not provide a way to track when a particular data transfer specified by a data descriptor is complete because a single shared DMA counter (616) may be utilized, and therefore decremented, for data transfer operations specified by multiple data descriptors. Because the shared DMA counters (616) in the example of FIG. 6 are not utilized to track the completion of a data transfer in systems capable of direct memory access (‘DMA’) transfer completion notification according to the present invention, the origin DMA engine (197) disregards values for the shared DMA counters (616). The origin DMA engine (197) disregards values for the shared DMA counters (616) by disabling a counter underflow interrupt that occurs when a counter reaches zero or simply not setting the bits in the counter status register (614) when a counter reaches zero.

In transmitting messages, the DMA engine (197) of FIG. 6 selects the data descriptor (618) from the injection FIFO buffer (628) and transmits the message (614) specified by the selected data descriptor (618) through the data communications network (108) to the target compute node (604). The DMA engine (197) of FIG. 6 transmits the message (614) specified by the selected data descriptor (618) through the data communications network (108) to the target compute node (604) by encapsulating portions of the message (614) into data packets and injecting the data packets into transmission stacks (630, 632) in the communications hardware (636). The communications hardware (636) of FIG. 6 includes a static routing transmission stack (630) that stores packets for transmission through the data communications network (108) using static routing. The communications hardware (636) of FIG. 6 also includes a dynamic routing transmission stack (632) that stores packets for transmission through the data communications network (108) using dynamic routing.

The DMA engine (197) of FIG. 6 is also capable of DMA transfer completion notification according to embodiments of the present invention. The DMA engine is capable storing a data descriptor (706) for a message (601) to be sent in the injection FIFO buffer (628) and assigning an injection FIFO buffer sequence number to the data descriptor and storing the injection FIFO sequence number (704) for the data descriptor (706) in application storage (702) allocated to an application (158) sending the message (601). The DMA engine typically assigns an injection FIFO sequence number that is one integer greater than the injection FIFO sequence number of the previous data descriptor stored in the injection FIFO buffer. That is, the DMA engine typically increments the injection FIFO sequence numbers for each data descriptor stored in the injection FIFO buffer (628). To avoid ultimately assigning very large values for the injection FIFO sequence number, a maximum value may be established and in such cases after assigning an injection FIFO sequence number of the maximum value, the DMA rolls over and assigns as the next injection FIFO sequence number the lowest value available. Such a DMA engine may then continue to increment the injection FIFO sequence numbers by a value of one.

The DMA engine (197) of FIG. 6 is also capable of determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer. If the data descriptor for the application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer then the message has not yet been sent and a processor core is not interrupted or notified. However, if the data descriptor for the application message to be sent to a target compute node is not currently in an injection first-in-first-out (‘FIFO’) buffer then the application message has been sent and the DMA engine of FIG. 6 is capable of notifying a processor core on the origin DMA engine that the message has been sent.

For further explanation, FIG. 7 sets forth a flow chart illustrating an exemplary method for DMA transfer completion notification. The method of FIG. 7 includes storing (700), by the DMA engine (197), the data descriptor (706) for the message (601) in the injection FIFO buffer (628). As mentioned above, a data descriptor (706) for a message (601) is a data structure that describes the message for transmission and specifies transmission data related to that message. For example, a data descriptor may describe the address of the buffer storing the message, the size of the message, the type of the message, packet headers to be used in transmitting the message, and other information as will occur to those of skill in the art. The data descriptor (706) may be created by a messaging module for the origin node and created in response to the invocation of an API by an application and passed for storage in an injection FIFO buffer to the DMA engine.

The method of FIG. 7 also includes assigning (701), by the DMA engine (197), an injection FIFO buffer sequence number (704) to the data descriptor (706). The DMA engine typically assigns an injection FIFO sequence number that is one integer greater than the injection FIFO sequence number of the previous data descriptor stored in the injection FIFO buffer. That is, the DMA engine typically increments injection FIFO sequence number the injection FIFO sequence numbers for each data descriptor stored in the injection FIFO buffer (628). To avoid incrementing to very large values for the injection FIFO sequence number, a maximum value may be established and in such cases after assigning an injection FIFO sequence number of the maximum value, the DMA rolls over and assigns as the next injection FIFO sequence number the lowest value available. Such a DMA engine may then continue to increment the injection FIFO sequence numbers by a value of one.

Assigning (701), by the DMA engine (197), an injection FIFO buffer sequence number (704) to the data descriptor (706) may be carried out by storing the injection FIFO sequence number (704) for the data descriptor (706) in application storage (702) allocated to an application (158) sending the message (601).

The method of FIG. 7 also includes determining (708), by an origin DMA engine (197) on an origin compute node, whether a data descriptor (706) for an application message (601) to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer (628) in dependence upon a sequence number (704) previously associated with the data descriptor (706), the total number (710) of descriptors currently in the injection FIFO buffer (628), and the current sequence number (712) for the newest data descriptor stored in the injection FIFO buffer. Determining whether a data descriptor for the message is currently in the injection FIFO buffer may be carried out by calculating the sequence number of the oldest data descriptor stored in the injection FIFO buffer; and determining whether the sequence number for the data descriptor for the message is less than the sequence number of the oldest data descriptor stored in the injection FIFO buffer as discussed below with reference to FIG. 8.

If the data descriptor for the message is currently in the injection FIFO buffer, then the message has not yet been sent and therefore the processor core is not notified or interrupted because the message has not yet been sent. However, if the data descriptor for the message is not currently in the injection FIFO buffer, then the message has been sent and the method of FIG. 7 includes notifying (714) a processor core on the origin compute node that the message has been sent if the data descriptor (706) for the message (601) is not currently in the injection FIFO buffer (628). Notifying (714) a processor core on the origin compute node may be carried out by invoking a callback function or interrupt.

As mentioned above, in some embodiments of the present invention the data descriptor specifies a shared DMA counter. In such embodiments, notifying a processor core on the origin DMA engine that the message has been sent includes disregarding a value for the shared DMA counter. Because the shared DMA counters are not utilized to track the completion of a data transfer in systems capable of direct memory access (‘DMA’) transfer completion notification according to the present invention, the origin DMA engine (197) disregards values for the shared DMA counters (616). The origin DMA engine (197) disregards values for the shared DMA counters (616) by disabling a counter underflow interrupt that occurs when a counter reaches zero or simply not setting the bits in the counter status register (614) when a counter reaches zero.

As mentioned above, DMA transfer completion notification according to the present invention includes determining, by the origin DMA engine, whether a data descriptor for the message is currently in the injection FIFO buffer. For further explanation, therefore, FIG. 8 sets forth a flow chart illustrating an exemplary method for determining, by the origin DMA engine, whether a data descriptor for the message is currently in the injection FIFO buffer. The method of FIG. 8 includes calculating (802) the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628). Calculating (802) the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628) may be carried out by identifying the sequence number of the newest data descriptor in the injection FIFO buffer, determining the number of data descriptors in the injection FIFO, subtracting the number of data descriptors in the injection FIFO from the newest sequence number, and adding 1. That is, calculating (802) the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628) may be carried out according to the following formula: Θ_(old)=Θ_(new)−Σ_(descriptors)+1 wherein:

-   Θ_(old)=the sequence number of the oldest data descriptor in the     injection FIFO buffer; -   Θ_(new)=the sequence number of the newest data descriptor in the     injection FIFO; and -   Σ_(descriptors)=the total number of data descriptors in the     injection FIFO buffer.

Determining whether a data descriptor for the message is currently in the injection FIFO buffer according to the method of FIG. 8 also includes determining (806) whether the sequence number (808) for the data descriptor (706) for the message (614) is less than the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628). If the sequence number (808) for the data descriptor (706) for the message (614) is not less than the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628) then the message has not been sent (716) and the data descriptor for the message is currently in the injection FIFO buffer. If the sequence number (808) for the data descriptor (706) for the message (614) is less than the sequence number (804) of the oldest data descriptor (618) stored in the injection FIFO buffer (628) then the message has been sent and the data descriptor for the message is not currently in the injection FIFO buffer and the method of FIG. 8 includes notifying (714) a processor core on the origin DMA engine that the message has been sent.

Exemplary embodiments of the present invention are described largely in the context of a fully functional computer system for direct memory access (‘DMA’) transfer completion notification. Readers of skill in the art will recognize, however, that the present invention also may be embodied in a computer program product disposed on computer readable media for use with any suitable data processing system. Such computer readable media may be recordable media for machine-readable information, including magnetic media, optical media, or other suitable media. Examples of recordable media include magnetic disks in hard drives or diskettes, compact disks for optical drives, magnetic tape, and others as will occur to those of skill in the art. Persons skilled in the art will immediately recognize that any computer system having suitable programming means will be capable of executing the steps of the method of the invention as embodied in a program product. Persons skilled in the art will recognize immediately that, although some of the exemplary embodiments described in this specification are oriented to software installed and executing on computer hardware, nevertheless, alternative embodiments implemented as firmware or as hardware are well within the scope of the present invention.

It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims. 

1. A computer-implemented method for direct memory access (‘DMA’) transfer completion notification, the method comprising: determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer, and notifying a processor core on the origin compute node that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.
 2. The method of claim 1 wherein determining, by the origin DMA engine, whether a data descriptor for the message is currently in the injection FIFO buffer further comprises: calculating the sequence number of the oldest data descriptor stored in the injection FIFO buffer; and determining whether the sequence number for the data descriptor for the message is less than the sequence number of the oldest data descriptor stored in the injection FIFO buffer.
 3. The method of claim 1 further comprising: storing, by the DMA engine, the data descriptor for the message in the injection FIFO buffer; and assigning, by the DMA engine, an injection FIFO buffer sequence number to the data descriptor.
 4. The method of claim 3 wherein assigning, by the DMA engine, an injection FIFO buffer sequence number to the data descriptor further comprises storing the injection FIFO sequence number for the data descriptor in application storage allocated to an application sending the message.
 5. The method of claim 1 wherein the data descriptor specifies a shared DMA counter, and notifying a processor core on the origin DMA engine that the message has been sent further comprises disregarding a value for the shared DMA counter.
 6. The method of claim 1, wherein the origin compute node and the target compute node are comprised in a parallel computer, the parallel computer comprising a plurality of compute nodes connected for data communications through a data communications network, the data communications network optimized for point to point data communications.
 7. An origin compute node capable of direct memory access (‘DMA’) transfer completion notification, the compute node comprising a direct memory access (‘DMA’) controller, an origin DMA engine installed upon the DMA controller, a computer processor, computer memory operatively coupled to the computer processor, the DMA controller, and the origin DMA engine, the computer memory having disposed within it computer program instructions performing the steps of: determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated with the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer; and notifying a processor core on the origin compute node that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.
 8. The compute node of claim 7 wherein computer program instructions performing the step of determining, by the origin DMA engine, whether a data descriptor for the message is currently in the injection FIFO buffer further comprise computer program instructions performing the steps of: calculating the sequence number of the oldest data descriptor stored in the injection FIFO buffer; and determining whether the sequence number for the data descriptor for the message is less than the sequence number of the oldest data descriptor stored in the injection FIFO buffer.
 9. The compute node of claim 7 wherein the computer memory also has disposed within it computer program instructions performing the steps of: storing, by the DMA engine, the data descriptor for the message in the injection FIFO buffer; and assigning, by tile DMA engine, an injection FIFO buffer sequence number to the data descriptor.
 10. The compute node of claim 9 wherein computer program instructions performing the step of assigning, by the DMA engine, an injection FIFO buffer sequence number to the data descriptor further comprise computer program instructions performing the steps of storing the injection FIFO sequence number for the data descriptor in application storage allocated to an application sending the message.
 11. The compute node of claim 7 wherein the data descriptor specifies a shared DMA counter, and computer program instructions performing the step of notifying a processor core on the origin DMA engine that the message has been sent further comprise computer program instructions performing the step of disregarding a value for the shared DMA counter.
 12. The compute node of claim 7, wherein the origin compute node is comprised in a parallel computer, the parallel computer comprising a plurality of compute nodes connected for data communications through a data communications network, the data communications network optimized for point to point data communications.
 13. A computer program product for direct memory access (‘DMA’) transfer completion notification, the computer program product disposed upon a non-transitory computer readable storage medium, the computer program product comprising computer program instructions performing the steps of: determining, by an origin DMA engine on an origin compute node, whether a data descriptor for an application message to be sent to a target compute node is currently in an injection first-in-first-out (‘FIFO’) buffer in dependence upon a sequence number previously associated With the data descriptor, the total number of descriptors currently in the injection FIFO buffer, and the current sequence number for the newest data descriptor stored in the injection FIFO buffer, and notifying a processor core on the origin compute node that the message has been sent if the data descriptor for the message is not currently in the injection FIFO buffer.
 14. The computer program product of claim 13 wherein computer program instructions performing the step of determining, by the origin DMA engine, whether a data descriptor for the message is currently in the injection FIFO buffer further comprise computer program instructions performing the steps of: calculating the sequence number of the oldest data descriptor stored in the injection FIFO buffer, and determining whether the sequence number for the data descriptor for the message is less than the sequence number of the oldest data descriptor stored in the injection FIFO buffer.
 15. The computer program product of claim 13 further comprising computer program instructions performing the steps of: storing, by the DMA engine, the data descriptor for the message in the injection FIFO buffer; and assigning, by the DMA engine, an injection FIFO buffer sequence number to the data descriptor.
 16. The computer program product of claim 15 wherein computer program instructions performing the step of assigning, by the DMA engine, an injection FIFO buffer sequence number to the data descriptor further comprise computer program instructions performing the step of storing the injection FIFO sequence number for the data descriptor in application storage allocated to an application sending the message.
 17. The computer program product of claim 13 wherein the data descriptor specifies a shared DMA counter; and computer program instructions performing the step of notifying a processor core on the origin DMA engine that the message has been sent further comprise computer program instructions perform the step of disregarding a value for the shared DMA counter. 